Cementing procedure for avoiding mud channeling

ABSTRACT

When cementing a pipe section (such as a production casing or liner) in a well, problems due to mud channeling are avoided by displacing the drilling fluid (before running-in the pipe section) and replacing the drilling fluid with a subsequently solidifiable liquid that remains pumpable while the pipe section is being installed and the cement slurry is being injected, and then solidifies, during or after, the solidifying of the cement.

UniteclStates Patent [19 1 Bruist Feb. 4, 1975 1 1 CEMENTING PROCEDUREFOR AVOIDING MUD CHANNELING [75] Inventor: Edmond H. Bruist,'NewOrleans,

[73] Assignee: Shell Oil Company, Houston, Tex.

[22] Filed: Mar. 27, 1974 21 Appl. No.: 455,171

3,291,211 12/1966 Savins et al 166/285 3,688,845 9/1972 Messenger....166/291 3,749,173 7/1973 Hill et al. 166/291 3,799,874 3/1974 Parker166/291 X Primary ExaminerStephen J. Novosad [57] ABSTRACT Whencementing a pipe section (such as a production casing or liner) in awell, problems due to mud channeling are avoided by displacing thedrilling fluid (before running-in the pipe section) and replacing thedrilling fluid with a subsequently solidifiable liquid that remainspumpable while the pipe section is being installed and the cement slurryis being injected, and then solidifies, during or after, the solidifyingof the cement.

9 Claims, 3 Drawing Figures CEMENTING PROCEDURE FOR AVOIDING MUDCHANNELING BACKGROUND OF THE INVENTION The invention relates to animproved process for cementing a pipe section in a well (e.g.,positioning a casing or liner in the borehole, injecting a slurry ofcement into the annular space around such a pipe section and allowingthe cement to solidify).

Procedures forcementing casings or other pipe sections in wells havebeen usedfor years. Numerous proposals have been made for avoiding theproblems referred to as mud channeling problems. For example, asdisclosed in US. Pat. No. 2,582,909, filed in 1947, it is known thatmaterials such as the dissolved calcium in acement slurry tend to reactwith and gel or solidify any contacted portions of an aqueous drillingfluid that contains clay that was either added to increase the viscosityof the fluid and/or was entrained during the drilling operation. While aslurry of cement is being flowed into the annular space around a casingin a well, the precipitated, gelled, or otherwise immobilized portionsof the drilling fluid tend to be by-passed by the slurry. The by-passedportions then become lumps or stringers amounting to mud channels withinthe sheath of solidified cement that surrounds the casing.

When a cemented-in portion of casing has been perforated in order toestablish fluid communication with a selected reservoir zone, fluidsinjected into or produced from the reservoir tend to move through oraround any by-passed drilling fluid materials in the mud channels. Suchflows destroy the zonal isolation the cement is intended to provide andmay result in a need for an expensive remedial cement squeezingoperation and/or cause aloss of the well.

As indicated by US. Pat. No. 2,848,05 1 in 1954, the continued existenceof mud channeling problems led to a proposal that the cement slurrybeing injected into the annular space around the casing be preceded by aslug of an inert slurry that has improved properties relative to thoseof a slurry of cement with respect to not causing the gelling ofadrilling fluid and/or with respect to displacing a gelled drilling fluidin order to provide a better mud displacement. However, as indicated byUS. Pat. No. 3,749,173; in 1972, the continued existence of mudchanneling problems led to a proposal that such a spacer fluid fordisplacing the drilling fluid from the annular space around the casingbe an organic liquid that is gelled with a metal salt of an aliphaticsubstituted orthophosphate ester.

SUMMARY OF THE INVENTION cantly less than the borehole diameter isextended to near the bottom of a drilling-fluid-containing portion of awell borehole into which a pipe section is to be installed. Fluid isflowed through the pipe string to displace substantially all of thedrilling fluid out of that portion of the borehole. Substantially theonly fluid which is left in that portion of the borehole is asubsequently solidiflable fluid that (a) has a density at leastsubstantially equalling that of the drilling fluid which was displaced,(b) has a chemical composition adapting it to remain pumpable whileremaining at the borehole temperature until the pipe section has beeninstalled and a cement slurry has been inflowed, and (c) subsequentlysolidifies to form a substantially imperm eable solid.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic cross section of aborehole in which a casing has been cemented by the procedures of theprior art.

FIGS. 2 and 3 are schematic illustrations of portions of boreholes andmaterials in them during different stages of practicing the presentinvention.

DESCRIPTION OF THE INVENTION FIG. 1 shows a casing 1 that was cementedin a borehole 2 by a prior art cementing procedure. Typically, such aprocedure results in a sheath of solidified cement 3, that is permeatedby channels of by-passed drilling fluid 4. Such mud channels are apt tobe formed during substantially any attempt to displace a drilling fluidout of the annular space around a pipe section such as a casing with orwithout the use of a mud displacing fluid between the cement and thedrilling fluid. Several factors are involved.

A time interval exceeding one to three days is usually required forremoving a drilling string from the well and installing a casing orliner in the portion of the borehole in which it is to be cemented. Adrilling fluid is designed to maintain selected specific gravity andviscosity properties while it is being circulated into and out of aborehole. When a drilling fluid is kept static at the relatively hightemperature near the bottom of the borehole, e.g. is not circulatedwhile the drill string is removed and a casing is inserted, it tends tobecome exceedingly viscous.

The annular space between a casing or liner and a borehole wall isgenerally relatively small. For example, the spaces around a 7-inchcasing in a 9% inch borehole, or a 9% inch casing in a 12% inch boreholeprovide average clearances of less than 1 /2 inch. The boreholes areseldom perfectly straight and vertical. Where the boreholes are curved,the casings are forced closer to one side of the borehole, even wherecasingcentralizers are used. In general, the annular space around a pipesection being cemented is a tortuous meandering space that containsnumerous restrictions. This almost ensures that an inflowing cementslurry will bypass the drilling fluid to leave mud channels in thesolifified cement. Where one portion of the path is restricted, most orall of the inflowing slurry is diverted into another portion.

Field experience has indicated that the main problem in obtainingsatisfactory primary cementation is still caused by the incompletedisplacement of the drilling fluid by the cement slurry. The resultingmud channeling may occupy a large percentage of the annular volumebetween the casing and the formation. The severity of the channelingdepends upon the condition of the borehole and the cementing techniquesthat are applied. In order to minimize the mud channeling problem,numerous relatively expensive and/or difficult and time-consumingcementing techniques are commonly employed. Such techniques seek toensure that the casing and/or liner by (l) properly centralized (2)equipped with scratchers (3) reciprocated or rotated during thecementing; in addition, (4) the borehole should be free of washed-outzones, (5) the cement slurry should be pumped in with either relativelyhigh or relatively low pumping rates in order to induce either aturbulent flow or a plug flow, (6) the cement slurry and drilling fluidshould be separated by a suitable and sufficient spacer fluid, and (7)the properties such as viscosity, weight, and water-loss, of the cementslurry and those of the drilling fluid should be compatible and becarefully controlled. Even when all of these conditions are stringentlyadhered to, there is no assurance that a satisfactory primarycementation will be obtained. In practice, the borehole situation isoften far less favorable than desired, and many of the specifiedconditions are not, and often cannot, be met.

The present invention'is, at least in part, based on a discovery thatthe mud channeling problems can advantageously be avoided by utilizing aparticular sequence of steps that leave the borehole filled with asubsequently solidifiable fluid at the time the casing is inserted intothe borehole. The drilling fluid is displaced before, rather than after,the pipe section to be cemented is placed within the boreholes. It isparticularly desirable that the borehole section from which the drillingfluid is displaced extend at least substantially through the objectivezone of the subterranean reservoir into which the well is to becompleted. This ensures that substantially all of the fluid left in thetreated portion of the borehole is adapted to remain pumpable for atime, preferably about two days or more, sufficient to ensure that itremains pumpable throughout the removal of the drill pipe, or workstring, the installation of the pipe section to be cemented, and theinjection of the cement slurry. The displacing of the drilling fluidbefore the installing of the pipe section to be ccmented also provides aconvenient opportunity for removing substantially all of the filter cake(deposited by the drilling fluid) from the borehole wall. As a result ofthe drilling fluid replacement, the cement slurry can be pumped into theannular space around the pipe section to be cemented, and theresolidified, with a minimum of time and trouble and with an assurancethat the sheath formed by the solidifying of both the cement and anybypassed portions of the subsequently solidifiable fluid will befluid-tight.

The subsequently solidifiable fluid used in the present invention cancomprise substantially any pumpable fluid (solution and/or slurry) that(a) has a density at least substantially equalling that of the drillingfluid that is displaced from the borehole, and (b) has a chemicalcomposition adapting it to remain pumpable until a pipe section has beeninstalled and a cement slurry inflowed and then form a substantiallysolid material that is, effectively, impermeable and immobile. Such afluid can contain relatively slowly reactive components that complete afluidsolidifying reaction in response to an appropriate time-temperatureexposure. Alternatively, or additionally, it can contain potentiallyreactive components that initiate a relatively slow fluidsolidifyingreaction when they are contacted with materials that diffuse into themwhile they are maintained in contact with a cement slurry. Examples ofsuitable subsequently solidifiable fluids include; aqueous liquids thatare thickened with dispersed clays and contain lime and a caustic inproportions such that they are relatively slowly time-solidifying fluidswith respect to temperatures above about 200F; aqueous liquidsuspensions containing inert particles mixed with particles ofmaterials, such as raw cement or pozzolan or the like, that tend to berelatively slowly solidified when they are kept in contact with a cementslurry, such as the raw cement or pozzolancontaining slurries describedin U.S. Pat. No. 2,848,051; a time solidifying metal hydroxide-gelledfluid containing enough ungelled metal salt and pH-altering reactant tobecome subsequently solidified as the pH is changed, such as thosedescribed in the co-pending patent application by .I. A. l-lerce and R.N. Tuttle, Ser. No. 424,397, filed Dec. 13, 1973 (the disclosures ofwhich are incorporated herein by cross-reference); and the likecomposition. Particularly suitable subsequently solidifiable fluidsinclude aqueous liquid solutions of water-soluble amphoteric metals,water soluble bases, and water soluble pH-altering reactants, in whichsolutions are suspended fine solids in proportions such that thesolution initially has a suitable viscosity and density and, with timeat the temperature of the zone in which a pipe section is to becemented, becomes a substantially impermeable and substantially solidmaterial. For example, such a suspension comprises water containingenough dissolved aluminum salt to provide about 2 moles per liter ofaluminum ions, enough dissolved base to provide a solution equivalent toabout a 4 molar sodium hydroxide solution, and an amount of carboxylicacid amide equivalent to form about l2 moles per liter of ammonia (withthe amide composition being correlated with the borehole temperature sothat the faster reacting materials, such as formamide or acetamide, areused for lower temperatures than are the slower reacting materials, suchas urea) and from about 20 to 40 percent by weight of the aqueous liquidof a finelydivided substantially inert solid material, such as silicaflour.

FIG. 2 shows an initial stage of a casing cement operation utilizing thepresent invention. Borehole 6 contains a drilling fluid 7 within aportion of the borehole in which a casing is to be installed. Asindicated by the arrows, a subsequently solidifiable fluid 8 is beingflowed through a pipe string 9 (such as a drill string or a work string)that is equipped with a drill bit 10, and a filter cake removing device11 and is extended to near the bottom of that portion of the borehole.

Where desirable, a slug of an inert mid-displacing spacer, such as thosedescribed in U.S. Pat. Nos. 2,848,051 or 3,749,173, or the like (notshown), can be injected ahead of the subsequently solidifiable fluid.The density of the one or more fluids that are inflowed through such apipe string to displace and/or replace the drilling fluid should besubstantially equal to that of the displaced drilling fluid, in order toavoid a tendency for gravity segregation within the borehole, and tomaintain a hydrostatic pressure substantially equalling that provided bythe drilling fluid.

The fluid-inflowing pipe string 9 preferably has an outer diameter thatis significantly less than the inner diameter of the borehole, such as adrilling string having an outer diameter of about 4 inches in a boreholehaving a diameter of at least about 9 inches. The annular space aroundsuch a pipe string is, in effect, a large diameter conduit that offerssubstantially no restriction to fluid flowing at the flow rates that arefeasible and desirable relative to displacing a drilling fluid from aborehole. Such a pipe string is preferably equipped with at least onefilter-cake removing device, such as a drill bit, and/or properly spacereamers, fluid jets, a flow restricting enlargement of the pipe stringouter diameter, or the like, for mechanically and/or hydraulicallystripping off any filter cake (or mud cake) that was formed along thewall of the borehole. A substantially complete displacement of thedrilling fluid can readily be effected by mechanically manipulating thepipe string assembly to enhance the filter-cakestripping action and/orto ensure the completeness of the displacement of the drilling fluid.Such a mechanical manipulation can advantageously include the rotatingof the pipe string assembly and/or the making ofone or more short tripsthrough the portion of the borehole in which mud channels in the cementsheath must be avoided, i.e. the objective reservoir zone.

FIG. 3 shows a later stage of a casing cementing operation of thepresent type. A string of casing 12 is positioned within a portion ofthe borehole that contains the subsequently solidifiable fluid 8. Acement slurry 13 is flowed through the casing and into the annulusaround the casing, as shown by the arrows. Since the fluid in theannulus is a subsequently solidifiable fluid, no mud channels will beformed in the resulting sheath of solidified cement due to any portionsthat are bypassed by the inflowing cement slurry. The bypassed portionswill solidify and ensure the fluid-tight integrity of all portions ofthe cement sheath.

In certain situations the present process can be simplified,particularly where the portion of the borehole into which a pipe sectionis to be cemented is relatively shallow and/or is unlikely to besubjected to high stress. The subsequently solidifiable fluidcomposition is selected and/or adjusted so that, at the temperature ofthe zone in which the pipe section is to be installed, the fluid remainspumpable for at least about a day and then solidifies (in the absence ofany contact with a cement slurry) within not more than about days andbecomes substantially solid and impermeable. The drilling fluid isdisplaced by inflowing that subsequently solidifiable fluid through arelatively slender pipe string. The latter is replaced by the pipesection to be installed. And, the subsequently solidifiable fluid isallowed to solidify to provide the sealing and stabilizing bond betweenthe pipe section and the borehole wall.

What is claimed is: l. A process for installing a pipe section that isto be cemented within a drilling-fluid-containing portion of a boreholeof a well, which process comprises;

extending a relatively slender pipe string having an outer diametersignificantly less than the borehole diameter into the borehole until itextends to at least near the bottom of the portion of the borehole inwhich the pipe section is to be cemented;

flowing fluid through the relatively slender pipe string to displacesubstantially all of the drilling fluid out of that portion of theborehole",

filling that portion of the borehole substantially completely with asubsequently solidifiable fluid that has a density at leastsubstantially equalling that of the displaced drilling fluid and has achemical composition that adapts it to remain pumpable, while remainingat the borehole temperature until the pipe section has been installedand a cement slurry has been inflowed, and then become substantiallysolid and impermeable;

removing the relatively slender pipe string and installing the pipesection to be cemented, and flowing a cement slurry into the annularspace around the pipe section and allowing the solidification of boththe cement and any subsequently solidifiable fluid that is present inthat annular space. 2. The process of claim 1 in which the relativelyslender pipe string is mechanically manipulated to enhance the removingof a filter cake from the borehole wall and the displacing of thedrilling fluid.

3. The process of claim 2 in which the relatively slender pipe string isequipped with a mechanical device for removing a filter cake.

4. The process of claim 1 in which the drilling fluid is displaced byflowing the subsequently solidifiable fluid into contact with thedrilling fluid.

5. The process of claim 1 in which the subsequently solidifiable fluidis adapted to remain pumpable at the borehole temperature for at leastabout one day before becoming substantially solid and impermeable.

6. The process of claim 1 in which the subsequently solidifiable fluidis a metal hydroxide-thickened aqueous fluid containing unreacteddissolved metal salt, suspended particles, and a pH-altering reactant inproportions such that the fluid is subsequently converted to a materialthat is substantially impermeable and solid.

7. The process of claim 1 in which the temperature of the portion of theborehole in which the casing is to be installed is at least about F andthe subsequently solidifiable fluid is an aqueous fluid containingdispersed clay and lime and dissolved caustic in proportions such thatthe fluid subsequently becomes a substantially rigid solid material.

8. The process of claim 1 in which the subsequently solidifiable fluidis an aqueous liquid containing suspended particles of material that isslowly solidified when contacted by a cement slurry in proportions suchthat any cement slurry-bypassed portions of the fluid are subsequentlyconverted to material that is substantially impermeable and solid.

9. In a process in which a pipe section is installed within a portion ofa borehole that was filled with drilling fluid during the drilling ofthe well and the space between the pipe section and the borehole wall isfilled with fluid that is initially pumpable but is intended to laterbecome solid and impermeable, the improvement which comprises:

extending a relatively slender pipe string having an outer diametersignificantly less than the borehole diameter into the borehole until itextends to at least near the bottom of the portion of the borehole inwhich the pipe section is to be installed;

flowing fluid through the relatively slender pipe string to displacesubstantially all of the drilling fluid out of that portion of theborehole;

filling that portion of the borehole substantially completely with asubsequently solidifiable fluid that has a density at leastsubstantially equalling that of the displaced drilling fluid and has achemical composition that adapts it to remain pumpable, while remainingat the borehole temperature until the pipe section has been installed,and then become substantially solid and impermeable;

removing the relatively slender pipe string and installing the pipesection to be installed; and allowing the solidification of thesubsequently solidifiable fluid in the annular space around the pipesection.

1. A process for installing a pipe section that is to be cemented withina drilling-fluid-containing portion of a borehole of a well, whichprocess comprises; extending a relatively slender pipe string having anouter diameter significantly less than the borehole diameter into theborehole until it extends to at least near the bottom of the portion ofthe borehole in which the pipe section is to be cemented; flowing fluidthrough the relatively slender pipe string to displace substantially allof the drilling fluid out of that portion of the borehole; filling thatportion of the borehole substantially completely with a subsequentlysolidifiable fluid that has a density at least substantially equallingthat of the displaced drilling fluid and has a chemical composition thatadapts it to remain pumpable, while remaining at the boreholetemperature until the pipe section has been installed and a cementslurry has been inflowed, and then become substantially solid andimpermeable; removing the relatively slender pipe string and installingthe pipe section to be cemented, and flowing a cement slurry into theannular space around the pipe section and allowing the solidification ofboth the cement and any subsequently solidifiable fluid that is presentin that annular space.
 2. The process of claim 1 in which the relativelyslender pipe string is mechanically manipulated to enhance the removingof a filter cake from the borehole wall and the displacing of thedrilling fluid.
 3. The process of claim 2 in which the relativelyslender pipe string is equipped with a mechanical device for removing afilter cake.
 4. The process of claim 1 in which the drilling fluid isdisplaced by flowing the subsequently solidifiable fluid into contactwith the drilling fluid.
 5. The process of claim 1 in which thesubsequently solidifiable fluid is adapted to remain pumpable at theborehole temperature for at least about one day before becomingsubstantially solid and impermeable.
 6. The process of claim 1 in whichthe subsequently solidifiable fluid is a metal hydroxide-thickenedaqueous fluid containing unreacted dissolved metal salt, suspendedparticles, and a pH-altering reactant in proportions such that the fluidis subsequently converted to a material that is substantiallyimpermeable and solid.
 7. The process of claim 1 in which thetemperature of the portion of the borehole in which the casing is to beinstalled is at least about 150*F and the subsequently solidifiablefluid is an aqueous fluid containing dispersed clay and lime anddissolved caustic in proportions such that the fluid subsequentlybecomes a substantially rigid solid material.
 8. The process of claim 1in which the subsequently solidifiable fluid is an aqueous liquidcontaining suspended particles of material that is slowly solidifiedwhen contacted by a cement slurry in proportions such that any cementslurry-bypassed portions of the fluid are subsequently converted tomaterial that is substantially impermeable and solid.
 9. In a process inwhich a pipe section is installed within a portion of a borehole thatwas filled with drilling fluid during the drilling of the well and thespace between the pipe section and the borehole wall is filled withfluid that is initially pumpable but is intended to later become solidand impermeable, the improvement which comprises: extending a relativelyslender pipe string having an outer diameter significantly less than theborehole diameter into the borehole until it extends to at least nearthe bottom of the portion of the borehole in which the pipe section isto be installed; flowing fluid through the relatively slender pipestring to displace substantially all of the drilling fluid out of thatportion of the borehole; filling that portion of the boreholesubstantially completely with a subsequently solidifiable fluid that hasa density at least substantially equalling that of the displaceddrilling fluid and has a chemical composition that adapts it to remainpumpable, while remaining at the borehole temperature until the pipesection has been installed, and then become substantially solid andimpermeable; removing the relatively slender pipe string and installingthe pipe section to be installed; and allowing the solidification of thesubsequently solidifiable fluid in the annular space around the pipesection.